The present invention relates to machine tools in general, and more particularly to improvements in tool changing apparatus for use in machine tools, especially in circular grinding machines which operate with large grinding wheels.
Shortening of down times is sought in all types of modern high-speed machine tools which are designed to treat workpieces at a high frequency. Down times are unavoidable during an exchange of workpieces, frequently during dressing of grinding wheels in a grinding machine, and particularly during an exchange of tools. A reduction of down times during the exchange of tools in a high-speed grinding machine can be avoided by using a tool magazine and a tool changer which is operated in response to signals from the automatic control system of the machine so that the exchange of tools is integrated into automatic operation of the machine. A grinding wheel must be exchanged when the wear upon its working surface is so pronounced that it is necessary to insert a new grinding wheel, or when a particular workpiece or two or more successive workpieces must be treated by different grinding wheels. In the latter instance, the previously used grinding wheel is detached from the spindle and is placed into storage while one or more other grinding wheels are in actual use.
German Offenlegungsschrift No. 20 24 212 discloses a grinding wheel changing apparatus which is designed to transfer grinding wheels between a horizontal tool spindle and a magazine wherein the grinding wheels are temporarily stored in such orientation that the axis of rotation of each grinding wheel is substantially horizontal. Thus, the orientation of the grinding wheel remains unchanged during transfer from the spindle into the magazine or vice versa.
German Offenlegungsschrift No. 32 46 168 discloses a changing apparatus which manipulates the grinding wheels in the same way as the apparatus of the Auslegeschrift, i.e., the axis of each tool remains horizontal during transfer from the spindle as well as during storage in the magazine.
A drawback of the aforedescribed conventional tool changing apparatus is that the grinding wheels are stored in such positions that their axes are horizontal, i.e., that the grinding wheels are disposed in substantially vertical planes. The pores of grinding wheels which are removed from the spindle for temporary storage in a magazine are filled with coolant (normally water). The coolant tends to accumulate in the lower portion of a stationary grinding wheel, i.e., at a level below the horizontal axis of such grinding wheel, so that the grinding wheel is out of balance. When the grinding wheel is returned to and is affixed to the spindle, it must be gradually accelerated during a relatively long interval of time in order to allow for renewed uniform distribution of the cooling medium therein. This reduces the output of the grinding machine because the sum of intervals which are required for gradual acceleration of previously used grinding wheels during an entire shift contributes significantly to the total down time.
European patent application No. 00 58 913 discloses a magazine for a stack of superimposed grinding wheels which are held in such orientation that their axes are substantially vertical. Each grinding wheel is held by a discrete arm, and the arms are pivotable about a vertical axis in order to introduce the respective grinding wheels into or to withdraw the respective grinding wheels from the magazine. The arm which has withdrawn the respective grinding wheel from the magazine is thereupon caused to change the orientation of the grinding wheel through 90.degree. so that the axis of the grinding wheel is horizontal. The purpose of such change in orientation is to facilitate the task of the operator who is to transfer the grinding wheel into the machine. The European patent application does not disclose a tool changing apparatus and/or the manner of integrating such apparatus into an automatically operated grinding machine.